It’s Dewey.
And it matters. Take a look at a catalogue. Or a novel. Or any newspaper. Even this e-mail type compensates for a period. Here’s what it looks like with two: It’s a great big honking gap between sentences. It’s distracting, when the whole purpose of a double space when typing was to help with typewriter spacing, which didn’t automatically adjust for letter width and punctuation placement. See?
It’s not hard to change. I did it.